Power resistors are used in many applications to control electrical current or voltage by dissipating electrical power. A resistor may have either a fixed resistance or a variable resistance. Variable resistance power resistors are useful in applications in which the ability to conveniently adjust current flow or voltage is desirable. For example, variable resistance power resistors are used as as dimmer controls in lighting circuits, as components in automobile ignition circuits and test equipment, and in other applications.
All power resistors generate heat when they dissipate electrical power. The amount of heat generated is directly proportional to the amount of power dissipated. The heat must be removed from the resistor to prevent it from overheating and burning out. Heat removal is typically a function of the resistor design, with the rate of heat removal directly proportional to the thermal conductivity of materials used to construct the resistor and the amount of resistor surface area exposed to a cooling fluid. The cooling fluid is typically air.
Heat removal from a resistor can be enhanced by increasing the amount of surface area available for heat transfer. This is often done by adding fins to the resistor or by attaching the resistor to a finned heat sink. However, depending on the design of the resistor, fins can interfere with mechanical elements of the resistor. For example, fins could interfere with the operation of a variable single wire power resistor, which has a mechanical means for varying resistance. A conventional thick film power resistor, however, has a design which is compatible with cooling fins.
A conventional thick film power resistor typically comprises a heat sink attached to a ceramic plate, a thick film resistive circuit deposited or printed on the ceramic plate, and a moveable contactor which facilitates changing the resistance of the device. The heat sink, which may have fins, is typically made from a metal such as aluminum. The ceramic plate serves as an electrical insulator between the resistive circuit and the heat sink. The ceramic plate may be attached to the heat sink with mechanical means such as bolts or a spring clip or with a thermally conductive adhesive. If mechanical means are used, a thermal grease must be used to make thermal contact between the plate and the heat sink. If a thermally conductive adhesive is used, the adhesive itself is sufficient to make thermal contact between the plate and the heat sink.
Conventional thick film power resistors suffer from several drawbacks. First, they can be somewhat cumbersome to assemble. The ceramic plate is fragile and is subject to breakage if dropped during assembly. The plate may also be damaged if mechanical means are used to attach it to the heat sink. The need to use a thermally conductive grease or adhesive to make thermal contact between the plate and heat sink adds an assembly step and material cost. Second, although the ceramic plate is thermally conductive, it contributes a significant thermal resistance between the resistive circuit and the heat sink. The thermally conductive grease or adhesive also contribute a significant thermal resistance. Finally, the thermal grease tends to dry out over time, increasing its thermal resistance and impairing the thermal contact between the ceramic plate and heat sink.
In some environments, for example gas turbine engines, ceramic coatings have been plasma sprayed directly onto a base material to serve as a thermal barrier. Thermally conductive greases and adhesives are not needed with such coatings, and indeed, are incompatible with them. However, because such plasma-sprayed ceramic coatings have been used as thermal insulators, they have not been used in applications where an electrically insulating, but thermally conductive material is required.
Accordingly, it would be desirable to have a thick film variable power resistor which does not not have a breakable ceramic insulator and which does not require a thermally conductive grease or adhesive to conduct heat to a heat sink.